Furans 2-alkoxy- from

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

Diels-Alder reactions of oxazoles afford useful syntheses of pyridines (Scheme 53) (74AHC( 17)99). A study of the effect of substituents on the Diels-Alder reactivity of oxazoles has indicated that rates decrease with the following substituents alkoxy > alkyl > acyl >> phenyl. The failure of 2- and 5-phenyl-substituted oxazoles to react with heterodienophiles is probably due to steric crowding. In certain cases, bicyclic adducts of type (359) have been isolated and even studied by an X-ray method (87BCJ432) they can also decompose to yield furans (Scheme 54). With benzyne, generated at 0°C from 1-aminobenzotriazole and lead tetraacetate under dilute conditions, oxazoles form cycloadducts (e.g. 360) in essentially quantitative yield (90JOC929). They can be handled at room temperature and are decomposed at elevated temperatures to isobenzofuran. 

The reaction of a-bromoacetals with trimethylsilylenolates catalyzed by titanium tetrachloride provides /3-alkoxy-y-bromoketones, which are useful furan precursors (Scheme 33) (75CL527). A new synthesis of acylfurans is exemplified by the formation of the 3-acetyl derivative (146) by heating the brdmoalkene (145) (78JOC4596). 2,2-Dimethyl-3(2//)-furanone (148) has been synthesized from 3-hydroxy-3-methylbutan-2-one treatment with sodium hydride and ethyl formate gave the hydroxymethylene derivative (147), which was cyclized and dehydrated to the furanone (148) with hydrochloric acid (Scheme 34) (71TL4891). O... 

It is known that 3-aminobenzo[6]furan can be prepared from o-cyanophenols and a-halogenocarbonyl compounds with subsequent Thorpe cyclization (73JPR779). The extension of this synthesis to heteroatom substituted benzo[6]furans is straightforward (76JPR313). The reaction of potassium salts of 3-cyano-2-pyridones (e.g. 27) with a-halogenocarbonyl compounds (esters, ketones) yields 2-alkoxy-3-cyanopyridines which can be cyclized in the presence of sodium ethoxide to give 3-aminofuro[2,3-6]pyridines (Scheme 6). 

Furan underwent photocyclization reactions with 2-alkoxy-3-cyanopyridines <1999J(P1)171> and 2-alkoxynico-tinic acid esters <2002TL6103>, forming cage-like adducts, as shown in Scheme 44, that presumably resulted from a singlet [4-f4] cycloaddition followed by a triplet [24-2] cycloaddition. Reaction of 2-cyanofuran, however, provided the [44-4] product as the major isomer <2004TL4437>. 

The photoaddition of aldehydes or ketones to furan has been reported over the years. Griesbeck and his co-workers have established that the diaster-eoselectivity of the addition of benzaldehyde to furan is 212 1 for the formation of the exo and endo products (98) and (99). The study was extended to the addition of other carbonyl compounds (100) and the ratio of products from these additions is shown under the appropriate structure (101) and (102). L-Ascorbic acid and some of its derivatives (103) also undergo photochemical addition of aromatic aldehydes and ketones. With benzaldehyde and benzophenone the products obtained are the mixture of (104) and (105) with a preference for the formation of the former. The stereochemistry of the addition of the excited state carbonyl compound to ascorbic acid favours the path where the phenyl and the alkoxy groups are cis on the resultant oxetane. Benzaldehyde adds photochemically to the enamine (106) when the mixture is irradiated in acetonitrile solution. Three products are... 

Alkoxy-y-bromoketones (16), which in turn are available in good yields from a TiCU-promoted reaction of a-bromoacetals with silyl enol ethers, can be cyclized to furans in refluxing toluene yields are moderate to good (Scheme 7) <75CL527>. 

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